A range of social factors is known to influence immunological responses in laboratory primates. The present investigation examined the effects of two housing environments (stainless steel linked caging and modular gang cage for troupe housing) on lymphocyte subset cell counts in cynomolgus monkeys (Macaca fascicularis). The lymphocyte counts from prestudy blood samples from approximately 400 animals were evaluated. All T, B and NK cell counts were higher in animals housed in ganged enclosures compared with linked caged animals. High individual variability in lymphocyte subset counts during the prestudy phase was evident in animals from both housing environments. Therefore, the type of housing did not affect the variability. The data demonstrate that housing conditions can affect the immune response and furthermore, generally indicate enhanced immune responses for socially and environmental enrichment housed animals. This may have important implications in the management and treatment of primates for safety evaluation studies of biological test substances.